A UB3LYP and UMP2 theoretical investigation on unusual cation-pi interaction between the triplet state HB=BH (3 Sigma g-) and H+, Li +, Na +, Be 2+ or Mg 2+.

The nature of the unusual cation-pi interactions between cations (H(+), Li(+), Na(+), Be(2+) and Mg(2+)) and the electron-deficient B=B bond of the triplet state HB=BH (3 Sigma g-) was investigated using UMP2(full) and UB3LYP methods at 6-311++G(2df,2p) and aug-cc-pVTZ levels, accompanied by a comparison with 1:1 and 2:1 sigma-binding complexes between BH and the cations. The binding energies follow the order HB=BH...H(+) > HB=BH...Be(2+) > HB=BH...Mg(2+) >> HB=BH...Li(+) > HB=BH...Na(+) and HB=BH ((1)Delta(g))...M(+)/M(2+) > H(2)C=CH(2)...M(+)/M(2+) > HC identical with CH...M(+)/M(2+) > HB=BH (3 Sigma g-)...M(+)/M(2+). Furthermore, except for HB...H(+), the sigma-binding interaction energy of the 1:1 complex HB...M(+)/M(2+) is stronger than the cation-pi interaction energy of the C(2)H(2)...M(+)/M(2+), C(2)H(4)...M(+)/M(2+), B(2)H(2) ((1)Delta(g))...M(+)/M(2+) or B(2)H(2) (3 Sigma g-)...M(+)/M(2+) complex, and, for the 2:1 sigma-binding complexes, except for HBBe(2+)...BH, they are less stable than the cation-pi complexes of B(2)H(2) ((1)Delta(g)) or B(2)H(2) (3 Sigma g-). The atoms in molecules (AIM) theory was also applied to verify covalent interactions in the H(+) complexes and confirm that HB=BH (3 Sigma 3-) can be a weaker pi-electron donor than HB=BH ((1)Delta(g)), H(2)C=CH(2) or HC identical with CH in the cation-pi interaction. Analyses of natural bond orbital (NBO) and electron density shifts revealed that the origin of the cation-pi interaction is mainly that many of the lost densities from the pi-orbital of B=B and CC multiple bonds are shifted toward the cations.